XPath Addressing and Querying XML Markup Languages

XPath is an XML Path language defined by the W3C for addressing parts of an XML document, designed to be used by both XSLT and XPointer.
XPath gets its name from its use of a path notation as in URLs for navigating through the hierarchical structure of an XML document.

XML Path Language (XPath) Version 1.0 - Official W3C site describing this language for addressing xml parts of an XML document which is designed xml to be used by both XSLT and XPointer. xml (W3C Recommendation 16 November 1999)

XPath Package for OCaml - This package implements the full W3C XPath recommendation for the OCaml language; it may work with the output of PXP (XML parser), or with any implementation of the XPath data model.

Zvon: XPath Tutorial - A tutorial with several examples and pictures which xml can be downloaded for off-line use.